Our laboratories have recently developed a two element method for the induction of tolerance to murine islet allografts. The combination of small donor lymphocytes and an antibody directed against the co-activation molecule CD40 ligand (CD40L) has enabled us to achieve permanent islet allograft survival. This Project is intended to extend our initial observations to islet xenograft systems. We have published data showing that our two element protocol can substantially prolong the survival of both rat islet and skin xenografts in mice. In this proposal, we will 1) identify the optimal parameters for tolerance induction in normal mice engrafted with islets and skin of both rat and human origin and 2) identify the host immune response elements that permit xenograft survival in tolerized mice (Specific Aim #1). These model systems will then be investigated further in adoptive transfer studies using scid mouse recipients to identify the cellular mechanisms that permit xenograft survival in tolerized mice (Specific Aim #2). The mechanisms identified will be compared with those mechanisms found to be responsible for allograft tolerance in Project 1. This is an important objective because neither the identity nor the functional capabilities of the cellular elements which participate in xenograft vs. allograft rejection are known with certainty. Our third objective is to extend our two element tolerance induction protocol to the human immune system. We have developed a method for creating complete xenogeneic lymphohemopoietic chimeras using the NOD-scid mouse engrafted with human peripheral blood lymphocytes (Hu-PBL-NOD-scid mice). Hu-PBL-NOD-scid mice will be used to induce, tolerance in the engrafted human immune system to xenogeneic islets and skin (Specific Aim #3). Although directed primarily at the study of islet xenografts, the chimera technology developed for this Aim will also permit us 1) to study the induction of tolerance to human islet allografts in a small animal model system, and 2) to compare in this system PBLs from normal individuals with those obtained from patients with insulin-dependent diabetes mellitus (IDDM). Our overall goal is to induce xenotolerance in the human immune system using a two element method that can be adapted to clinical practice. We believe it realistic to hope that this method may eventually prove suitable for curing diabetes mellitus.